The present invention relates to the detection of trace amounts of gas species and, more particularly to laser-based systems for trace detection of nitrogen dioxide.
The precise monitoring of trace gases has applications in a wide range of fields, including the detection of environmental pollutants, tracking of contaminants in closed environmental systems, medical diagnostics, defense, and homeland security. In particular, real-time trace gas detection is important in environmental science (e.g., in atmospheric physics/chemistry concerning air quality control, for complying with Environmental Protection Agency (EPA) air quality regulations, safety monitoring, monitoring automobile emissions, as well as for the study of chemical reactions that environmental gases (such as NO2) undergo particularly in the presence of solar radiation). See for example, United States Environmental Protection Agency, “National Air Quality Status and Trends Through 2007, EPA-454/R-08-006”, United States Environmental Protection Agency, Washington D.C., (2008). In these applications the concentrations of pollutants typically range from parts per trillion (“ppt”) to parts-per-million (“ppm”) levels, and thus require techniques that are both highly sensitive and selective. Nitrogen oxides (NOx) are some of the more damaging of these pollutants and impact the environment and public health in multiple ways. They play important roles in the formation of photochemical smog, the formation of tropospheric ozone and the formation of acid rain.
Laser-based techniques for monitoring environmental pollutants in the air have many advantages over chemical and other techniques because of their ability to provide real-time monitoring capabilities with greater sensitivity and selectivity. In particular, the advent of quantum cascade lasers in the mid-infrared region covering the 4-24 μm range has provided an attractive source for investigating the spectroscopy of trace gases in the atmosphere and for constructing portable gas sensors. See Kosterev el al., “Chemical Sensing with pulsed QC-DFB lasers operating at 6.6 μm” Appl. Phys. B. 75, 351-357 (2002). Quantum cascade lasers provide access to the fundamental rotational-vibrational transitions of molecular species, thus offering improved sensitivity of several orders of magnitude over near-infrared diode laser based techniques that employ the detection of the overtones of these transitions. See Tittel et al., “Recent Advances in Trace Gas Detection Using Quantum and Interband Cascade Lasers,” Rev. of Laser Eng. 34, 275-282 (2006). Quantum cascade lasers have been used to detect several trace gasses (e.g., CO, CO2, NO, NH3, CH4 and N2O) with different spectroscopic techniques (e.g., laser absorption spectroscopy, Cavity Ringdown Spectroscopy, Integrated Cavity Output Spectroscopy) as described in the review articles by A. Kosterev e. al., “Application of quantum cascade lasers to trace gas analysis,” Appl. Phys. B 90, 165-176 (2008); and R. F. Curl et al., “Quantum cascade lasers in chemical physics” Chem. Phys. Lett. 487, 1-18 (2010). By employing an external cavity arrangement, a quantum cascade laser offers a narrow linewidth (Δν˜0.001 cm−1), highly stable and reproducible tunable continuous wave (“CW”) output, and a wide continuous tuning range; all of which are desirable for the study of complex spectra, as is the case in investigating trace gas components in the atmosphere. Examples of such prior studies include Wysocki et al., “Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications,” Appl. Phys. B 81, 769-777 (2005); and Karpf & Rao, “Absorption and wavelength modulation spectroscopy of NO2 using a tunable, external cavity continuous wave quantum cascade laser,” Appl. Opt. 48, 408-413 (2009).
Laser Absorption Spectroscopy is based on recording the change in intensity of laser radiation as it passes through a region containing the sample of interest. As the laser is tuned across a transition, the transmitted laser intensity is a function of frequency ν given by Beer's law:I(ν)=I0(ν)e−σ(ν)LN  (1)where I0 is the incident laser intensity, L is the optical path length, σ(ν) is the absorption coefficient and N is the concentration of the absorbing species in molecules per unit volume. See P. Werle, “Review of recent advances in laser based gas monitors,” Spectrochimica Acta A 54, 197-236 (1998). The absorbance of the species in the cell is defined by a=σ(ν)LN, and the maximum value of the absorbance of a line corresponds to amax.
The sensitivity of a spectrometer is often determined by taking the ratio of the amplitudes of the absorption line to that of the noise level. As a result, it is important to maintain the sample at a pressure that results in a narrow linewidth and large amplitude. Maintaining the sample at low pressure reduces collisional broadening. At high pressures, neighboring lines tend to overlap because of collisional broadening. For example, the full width at half maximum (“FWHM”) of the strongest NO2 transitions in the R-branch (ν˜1630 cm−1) at atmospheric pressure are ˜0.15 cm−1, which is much greater than the separation between the individual lines. See Rothman et al., “The HITRAN 2004 molecular spectroscopic database”, J. Quant. Spectrosc. Radiat. Transfer 96 139-204 (2005). It has been found that the optimal pressure for laser absorption techniques such as Tunable Diode Laser Absorption Spectroscopy (TDLAS) is in the 10-50 mbar range which provides the best balance between sensitivity and selectivity. See, J. M. Hollas, High Resolution Spectroscopy, Second Edition, (Wiley 1998). Standard techniques such as adding multiple scans may be used to enhance the signal-to-noise ratio.
Using a line's amplitude to detect a species, however, neglects the width of the line and as a result gives the same intensity for both broad and narrow lines with the same amplitude (see Hollas). This can be seen in FIG. 1 which shows a simulated NO2 absorption spectrum for a 25 ppm sample at pressures of 50 mbar, 200 mbar and 600 mbar (the simulation was generated using the HITRAN database of Rothman et al. and the SPECTRA software developed by Mikhailenko, et al., “Information-calculating system Spectroscopy of Atmospheric Gases. The structure and main functions,” Atmos. Oceanic Opt. 18, 685-695 (2005). The low pressure data are helpful to resolve closely spaced lines. Even though the 600 mbar spectrum represents the contribution of about twelve times more molecules than the 50 mbar spectrum, it shows only about a 25% enhancement in peak absorption: The majority of additional absorption manifests itself in the broadening of the lines. It is important to note, however, that due to the closely spaced and complex nature of the NO2 spectrum, the individual absorption features seen in the 50 mbar trace are not individual absorption lines, but instead are composed of closely spaced doublets or multiplets. For example, the absorption feature seen at 1630.23 cm−1 is actually comprised of two lines separated by 0.004 cm−1 resulting from the transitions between the (0 0 1)-(18 2 17) and (0 0 0)-(17 2 16) levels (note that the (ν1 ν3 ν3)-(N Ka Kc) level notation is used), see Rothman et al. Even though the different rotational-vibrational transitions are not resolved, the absorption features that comprise several transitions can be employed for the estimation of trace species.